fusefrontend_reverse: secure GetAttr against symlink races

...by using the OpenNofollow helper & Fstatat.

Also introduce a helper to convert from unix.Stat_t to
syscall.Stat_t.

Tracking ticket: https://github.com/rfjakob/gocryptfs/issues/165

internal/fusefrontend_reverse/rfs.go

@@ -7,6 +7,8 @@ import (
 	"path/filepath"
 	"syscall"
 
+	"golang.org/x/sys/unix"
+
 	"github.com/hanwen/go-fuse/fuse"
 	"github.com/hanwen/go-fuse/fuse/nodefs"
 	"github.com/hanwen/go-fuse/fuse/pathfs"
@@ -150,15 +152,13 @@ func (rfs *ReverseFS) GetAttr(relPath string, context *fuse.Context) (*fuse.Attr
 	if err != nil {
 		return nil, fuse.ToStatus(err)
 	}
-	absPath, _ := rfs.abs(pRelPath, nil)
-	// Stat the backing file
-	var st syscall.Stat_t
-	if relPath == "" {
-		// Look through symlinks for the root dir
-		err = syscall.Stat(absPath, &st)
-	} else {
-		err = syscall.Lstat(absPath, &st)
+	// Stat the backing file/dir using Fstatat
+	var st unix.Stat_t
+	dirFd, err := syscallcompat.OpenNofollow(rfs.args.Cipherdir, filepath.Dir(pRelPath), syscall.O_RDONLY|syscall.O_DIRECTORY, 0)
+	if err != nil {
+		return nil, fuse.ToStatus(err)
 	}
+	err = syscallcompat.Fstatat(dirFd, filepath.Base(pRelPath), &st, unix.AT_SYMLINK_NOFOLLOW)
 	if err != nil {
 		return nil, fuse.ToStatus(err)
 	}
@@ -169,7 +169,8 @@ func (rfs *ReverseFS) GetAttr(relPath string, context *fuse.Context) (*fuse.Attr
 		return nil, fuse.ToStatus(syscall.EOVERFLOW)
 	}
 	var a fuse.Attr
-	a.FromStat(&st)
+	st2 := syscallcompat.Unix2syscall(st)
+	a.FromStat(&st2)
 	// Calculate encrypted file size
 	if a.IsRegular() {
 		a.Size = rfs.contentEnc.PlainSizeToCipherSize(a.Size)

internal/syscallcompat/unix2syscall.go

@@ -0,0 +1,28 @@
+package syscallcompat
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+// Unix2syscall converts a unix.Stat_t struct to a syscall.Stat_t struct.
+// A direct cast does not work because the padding is named differently in
+// unix.Stat_t for some reason ("X__unused" in syscall, "_" in unix).
+func Unix2syscall(u unix.Stat_t) syscall.Stat_t {
+	return syscall.Stat_t{
+		Dev:     u.Dev,
+		Ino:     u.Ino,
+		Nlink:   u.Nlink,
+		Mode:    u.Mode,
+		Uid:     u.Uid,
+		Gid:     u.Gid,
+		Rdev:    u.Rdev,
+		Size:    u.Size,
+		Blksize: u.Blksize,
+		Blocks:  u.Blocks,
+		Atim:    syscall.Timespec(u.Atim),
+		Mtim:    syscall.Timespec(u.Mtim),
+		Ctim:    syscall.Timespec(u.Ctim),
+	}
+}